1. Field of the Invention
The present invention relates generally to volume holograms and more specifically to a volume hologram system and method for achieving desired beam separations.
2. Description of the Prior Art
Holograms are recordings of light intensity patterns created by the interference of two beams of mutually coherent light (the two beams are usually obtained by splitting a single laser beam). There are two major categories of holograms: transmissive and reflective. These two categories are further divided into physical types of holograms: surface relief holograms and volume holograms. Surface relief holograms can be recorded using photolithographic processes. The interference pattern is recorded as a periodic variation in thickness of the material while the refractive index of the material remains fixed.
In a volume hologram, the interference pattern is recorded as a periodic variation in the refractive index of the material while the thickness of the material remains fixed. The periodic variation in refractive index creates surfaces of peak refractive index within the material. These surfaces are referred to as Bragg surfaces. When the interference pattern is created by two plane waves or two waves with identical curvature at the hologram surface, the Bragg surfaces will be Bragg planes.
When the hologram is reilluminated by one of the original laser beams at an angle that results in maximum diffraction efficiency, the internal angle of the beam relative to the Bragg planes is referred to as the Bragg angle. The external angle of incidence at which the maximum diffraction efficiency occurs is also often referred to as the Bragg angle.
Recently, holograms have been used in optical data storage applications. These holograms allow beams of light to be separated for various purposes. These hologram systems include the following references: JP 1-13246, published Jan. 18, 1989; JP 1-55745, published Mar. 2, 1989; JP 1-55746, published Mar. 2, 1989; JP 1-86337, published Mar. 31, 1989; JP 1-86332, published Mar. 31, 1989; JP 1-146143, published Jun. 8, 1989; JP 1-55359, published Mar. 1, 1989; JP 63-25845, published Feb. 3, 1988; JP 62-219340, published Sep. 26, 1987; JP 61-123032, published Jun. 10, 1986; JP 1-18175, published Jan. 20, 1989; JP 61-42613, published Mar. 1, 1986; SU 1053056, published Nov. 7, 1983; EP 350,014, published Oct. 1, 1990; U.S. Pat. No. 5,013,107, issued May 7, 1991; and U.S. Pat. No. 4,497,534, issued Feb. 5, 1985.
Pending U.S. application Ser. No. 07/774,410 filed Oct. 10, 1991, assigned to the same assignee as the present application, teaches a volume hologram system which uses two holograms. A first volume hologram divides an incoming beam into two beams of desired polarization and/or intensity, and a second volume hologram adjusts the angular position of one beam with respect to the other.
A problem with using volume holograms has been that they are difficult to manufacture. The processing of the volume hologram may result in shrinkage of the hologram material and subsequent distortion of the Bragg planes. This makes it hard to accurately and consistently make a hologram system which achieves the desired beam separation at the desired angles.
What is needed is a volume hologram system which is easy to manufacture and consistently achieves the desired beam separations.